1. Field of the Invention
The present invention is broadly concerned with a process for treating organically contaminated waste water and is particularly suited for applications in which land space available for placement of a waste water treatment facility is limited and when the concentration of organic material is high.
2. Description of the Prior Art
The use of biological treatment systems for treatment of organically contaminated waste water is widespread. These systems utilize naturally occurring microorganisms, primarily bacteria, under controlled conditions to remove organic contaminants. The level of organic contamination is often measured by the amount of oxygen required by the bacteria to stabilize the wastewater, the biochemical oxygen demand (BOD). In essence, BOD denotes the organic material or xe2x80x9cfoodxe2x80x9d available to the bacteria.
Particular biological treatment systems are designed based upon the objectives for treatment of a particular waste water stream. The primary objective of any such system is to meet the minimum effluent treatment standards set by a particular industry or regulatory agency. Further design optimizations can be made in order to achieve various other secondary objectives such as minimizing energy, operating costs, capital expense, sludge production, and land space required.
Conventional biological treatment systems use activated sludge to provide bacteria for waste water treatment. The activated sludge generally comprises biosolids recycled from subsequent steps in the treatment process. In conventional activated sludge systems, waste water is fed into an aeration tank in which a large population of bacteria are grown in suspension. The bacteria are allowed to consume the organic material until an appropriate BOD level has been reached. The heavier biosolids are then separated from the effluent and a portion of the biosolids or sludge is recycled to the aeration tank. The remaining sludge is purged from the system and undergoes additional treatment and ultimate disposal. This process typically requires relatively large-capacity aeration tanks and large hydraulic retention times, often exceeding 24 to 36 hours.
In certain applications, land space available for placement of a waste water treatment facility is limited. Therefore, smaller aeration tanks with shorter hydraulic residence times are highly desirable. In order to employ such smaller tanks, it is necessary that BOD removal occur rapidly. It has been found that the most rapid BOD removal by microorganisms occurs during the log phase of microorganism growth. During the log phase, the microorganisms grow at a logarithmic rate, rapidly increasing their numbers. The log phase occurs when the dissolved oxygen is plentiful and the BOD is in excess of what the microorganisms can consume. In order to sustain this rapid growth rate, the microorganisms rapidly consume and metabolize the organic material removing it from the waste water.
Using conventional technology, it is difficult to operate a waste water system in the log phase of microorganism growth. If, during the use of small tanks with short residence times, bacteria were to remain in suspension during the treatment process, they would be hydraulically washed out of the system as fast as they could be grown, and a sufficiently large population of bacteria to rapidly reduce BOD levels could not be achieved. Therefore, it is desirable that only excess bacteria be flushed into the solids removal phase of the activated sludge system.
There is a need in the art for a waste water treatment process capable of effectively removing organic contaminants from waste water while utilizing smaller tanks than conventional activated sludge processes.
The present invention overcomes the problems noted above and provides a process for treating organically contaminated waste water having an incoming biochemical oxygen demand (BOD) level. Generally, a process according to the invention comprises initially treating the waste water in the presence of microorganisms (preferably bacteria) capable of utilizing the organic contaminants for microorganism replication, oxygen, and support media for the microorganisms, and causing the microorganisms to replicate and thereby reduce the incoming BOD level. A first portion of the microorganisms adhere to the support media, and a second portion of the microorganisms remain suspended in the waste water. Following the initial treatment step, at least part of the second suspended portion of the microorganisms are removed from the initially treated waste water, and after the removal step, the waste water is further treated in the presence of the microorganisms and oxygen to further reduce the BOD level.
The initial treatment step is preferably carried out during the log phase of microorganism growth. Maintaining log phase growth in systems having small tanks with short residence times is accomplished by growing bacteria on fixed or suspended support media within an aeration tank. Preferred media for use with the invention include synthetic resin pellets (preferably porous) providing large surface areas, suspended fabric segments, or any similar device which provides large surface areas. The microorganisms attach to the media and replicate to the point at which they saturate the media. The excess microorganisms which are grown xe2x80x9csloughxe2x80x9d off. Even though these microorganisms have detached from the media, they remain suspended in the tank system and continue to remove BOD until they are displaced from the aeration tank system by incoming waste water flow. The initial treatment step reduces the incoming BOD level by at least about 50% and preferably by at least about 75%, and operates with a very short hydraulic retention time, as low as one hour.
Typically, the organically contaminated waste water stream to be treated comprises a number of different organic compounds and materials. It is preferable to use of a variety of microorganism species to remove these compounds and materials from the waste water efficiently. As is well known in the waste water treatment industry, different microorganism species adapt to degrade the material which yields it the most energy. For example, some microorganisms will degrade the BOD of incoming waste water, while others degrade the end product of other microorganisms.
Oxygen may be supplied to the tank by any method known in the art, preferably aeration diffusers through which air is blown. Even more preferably, the aeration diffusers are arranged in a grid proximate the bottom of the aeration tank.
Conventional activated sludge systems tend to operate in what is commonly referred to as the endogenous phase of microorganism growth. In the endogenous phase, biological material is limited and the microorganisms begin to use their own cell mass to maintain their energy level, while others die. When the microorganisms, preferably bacterial, are in the endogenous phase, protozoa develop to consume some of the individual bacteria, while other bacteria produce a polymer which helps to hold the bacterial populations together in bacterial masses which tend to settle under quiescent conditions. This settling is typically performed in a conventional clarifier. Operation of an activated sludge system in the endogenous phase of microorganism growth is generally characterized by the use of large tanks having relatively long hydraulic retention times.
Because the initial treatment step operates in the log phase of microorganism growth, protozoa cannot develop and the suspended bacteria will not produce the polymer enabling them to agglomerate or floc. Preferably, the removal step of the present invention comprises subjecting the suspended portion of the microorganisms to dissolved air flotation in the presence of an added synthetic resin polymer to cause agglomerations of some of the microorganisms in the suspended portion and polymer. The synthetic resin polymer substitutes for the natural polymer produced by the microorganisms during the endogenous phase and aids in flocculation of the microorganisms. The agglomerations are thereafter removed from the initially treated waste water.
Dissolved air flotation is not the only method of removing suspended organisms from the initially treated waste water. In additional preferred emobidments, synthetic resin polymer may be added to the initially treated waste water to cause agglomerations of the microorganisms as in the dissolved air flotation method, however, other mechanical processes such as straining, screening, centrifuging, or a dewatering belt are used to separate the agglomerations from the initially treated waste water.
After the removal step, the waste water (which now contains primarily soluble BOD) is further treated in the presence of microorganisms and oxygen to further reduce the BOD level. Preferably, this further treating step comprises passing the waste water into a second stage activated sludge system. This second stage system is similar in design to a conventional activated sludge system thereby providing sufficient time for microorganism floc formation and conventional settling. By removing microorganisms after the first stage, not only is the BOD associated with these organisms removed, but space is freed up in the second stage to allow for production of solids from degradation of the soluble BOD. Since clarifiers following the second stage have a solids limit, the second stage aeration tanks would have to be much larger if they had to accommodate the solids from the first stage. A portion of the settled microorganisms from the second stage system are recycled back to the initial treatment step to assist in the treatment of additional quantities of waste water. Additional portions of the settled microorganisms are recycled to the second stage activated sludge system.
Because the further treatment step preferably operates similar to a more conventional activated sludge system, the further treatment step is preferably carried out for a period of time longer than the initial treatment step with the further treatment tank being larger than the initial treatment tank. The further treatment step is preferably carried out with hydraulic retention times as low as about 4 hours depending upon the concentration of organic material in the waste water. Furthermore, it is preferable that the initial and further treatment tanks be exposed to ambient air.
By removing a large portion of the biological mass after the initial treatment step, the need for additional oxygen and energy in the further treatment step is greatly reduced, leading to decreased costs for system operation.